12/10/2014
Kw, pH and pOH
Chemistry 12
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Chapter 14
Self‐ionization of Water:
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All aqueous solutions contain ions!
Even pure water contains a few ions that are produced by the dissociation of water molecules
Self‐ionization of Water continued….
At 25°C only two water molecules in a billion dissociate | Chemists have determined the concentration of H3O+ in pure water to be 1.0 x 10‐7 M and OH‐
ions in pure water is 1.0 x 10‐7 M as well.
| Because this dissociation is an equilibrium, we can write an equilibrium expression….called….
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Ion Product Constant of Water (Kw):
Kw = [H3O +][OH‐] = (1.0 x 10‐7 M)(1.0 x 10‐7 M) = 1.0 x 10‐14
(changes with temperature)
Note: water is not included because it is liquid!
***Important because if we know the concentration of either H3O + or OH‐,
we can solve for the other using Kw!!! [H3O+ ] and [OH‐ ] in aqueous solutions:
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Acid solution: [H3O +] is greater than 1.0 x 10‐7 M and [OH‐] less than 1.0 x 10‐7 M
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Basic solution: [OH ‐] is greater than 1.0 x 10‐7 M and [H3O +] less than 1.0 x 10‐7 M
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Neutral solutions concentrations of both equal 1.0 x 10‐7 M
Acid – Base Concentrations:
concentration
n (moles/L)
10-1
pH = 3
pH = 11
OH-
H3O+
pH = 7
10-7
H3O+
OH-
OH-
H3O+
10-14
[H3O+] > [OH-]
[H3O+] = [OH-]
acidic
solution
neutral
solution
[H3O+] < [OH-]
basic
solution
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pH Scale:
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Used to describe acids and bases quantitatively
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This scale measures the strength of an acid or base
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The concentrations H3O+ ions range from 1o M to g
10‐15 M
y
y
Thus, this scale uses logarithms to convert to more “user friendly” numbers
Since, the pH scale is logarithmic 1 unit pH change means tenfold change in the H+ ion concentration. pH Scale:
o
Uses Sørensen's pH definition:
Concentration of H+ is usually confined to 1 to 10‐14M range. o
o Thus pH scale contains values falling between 0 and 14. o For your own information: In some rare cases you may see pH lower than 0 or higher than 14, when the concentration of H+ take some extreme values. pH of Common Substances:
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pH and pOH:
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pH = ‐log10[H+] or pH = ‐log10[H3O+] (same thing!!)
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[H+] ] = [H
[H3O +] ] = 10
10‐pH
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pOH = ‐log10[OH‐]
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[OH‐] = 10‐pOH
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pH + pOH= 14
pH
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
[H1+]
[OH1-]
1 x 10-14
1 x 10-13
1 x 10-12
1 x 10-11
1 x 10-10
1 x 10-9
1 x 10-8
1 x 10-7
1 x 10-6
1 x 10-5
1 x 10-4
1 x 10-3
1 x 10-2
1 x 10-1
1 x 100
1 x 10-0
1 x 10-1
1 x 10-2
1 x 10-3
1 x 10-4
1 x 10-5
1 x 10-6
1 x 10-7
1 x 10-8
1 x 10-9
1 x 10-10
1 x 10-11
1 x 10-12
1 x 10-13
1 x 10-14
pOH
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
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Example 1:
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[H3O+] in a cola drink is about 5.0 x 10‐3 M. Calculate the pH of the drink. State whether the drink is acidic or basic.
y
pH = ‐log [H3O
pH log [H3O+]
pH= ‐log (5.0 x 10‐3) pH = 2.30
y
Therefore, the cola drink is acidic.
Example 2:
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Calculate the pH of a 0.0025M solution of Ca(OH)2.
y
y
y
Ca(OH)2 <‐> Ca2+ + 2OH‐1
0.0025M
0.005M
Kw = [H3O+] [OH‐] = 1.0 x 10‐14
(x) (0.005M) = 1.0 x 10‐14
x =[H3O+] = 2.0 x 10‐12
pH = ‐log [H3O+]
pH = ‐log (2.0 x 10‐12) pH = 11.7
Try it :
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Worksheet
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